1. Field
The present disclosure relates to a bulk acoustic (BAW) device and a method for switching ON and OFF a piezoelectric BAW device thin film resonator/filter comprising a piezoelectric film by varying the level of intrinsic polarization in the piezoelectric film.
2. Description of Related Art
A BAW device usually comprises a piezoelectric layer placed between two electrodes. When a radio frequency (RF) signal is applied across the device, a mechanical wave is produced in the piezoelectric layer. The thickness of the piezoelectric layer determines the resonant frequency, and the fundamental resonance occurs when the wavelength of the mechanical wave is about twice the thickness of the piezoelectric layer. As the thickness of the piezoelectric layer is reduced, the resonance frequency is increased.
Prior art BAW devices utilize piezoelectric materials that are strongly or permanently polarized in a preferred direction, where the piezoelectric materials may include AlN, ZnO, or lead zirconate titanate (PZT). The kind of PZT material used in prior art devices is a PZT having a crystal orientation which is predominantly a <111> orientation. See, for example, “Lead zirconate titanate thin films for microwave device applications” by S. Arscott, R. E. Miles, S. J. Milne in IEEE Proc.—Circuits Devices Syst., Vol. 145, No. 5, October 1988. See also “Piezoelectric Actuators and Ultrasonic Motors” by Kenji Uchino, Kluwer Academic Publishers, 1997. AlN and ZnO films are polarized during film fabrication itself, while the <111> oriented PZT films are polarized by applying a DC voltage across the material after fabrication. This provides devices that are permanently active or polarized. Such devices are not suitable for intrinsic switching.
Typical switching of RF signals in BAW devices is achieved by using electrostatic switches or PIN diodes. Those. switches are difficult to fabricate, possess a relatively large size and require actuation voltages, typically as high as 100 V. In addition, electrostatic switches suffer from low yield and reliability issues like stiction. PIN diode switches are very lossy, seriously degrading the device performance, and require additional electronic components and materials.